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Several websites define an ambidentate ligand as being monodentate. What about a ligand with say 3 atoms that can act as donor atoms, but due to the sterics of the ligand, only any two atoms can coordinate bond to an acceptor at any one time?

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    $\begingroup$ I don't really get your point there. "Ambidenticity" has little usefulness, pretty much only for simplest ligands. There are molecules able to coordinate in so many places that nobody counts. $\endgroup$ – Mithoron Feb 10 '19 at 22:30
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    $\begingroup$ If two atoms of the species can coordinate bond then the species is bidentate. A ligand is ambidentate if only one of two possible atoms can form a ligand, eg the linear $\ce{NCS^-}$ anion. There is no specific name for a ligand which is physically constrained so that only two of three possible ligand sites can bind. $\endgroup$ – MaxW Feb 10 '19 at 23:51
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If the ligand has 3 donor sites and it is able to show multiple denticity in different compounds, then it will be called as flexidentate ligand. But as per your question, if it is only showing denticity of 2 only it will be called as bidentate ligand.

Actually, this concept arose when arguments started for selecting donor sites for the " monodentate ligands " which were always showing denticity of only 1 in all of its compounds instead they had multiple donor sites.

This ambidentate case arises when a ligand can't use its two donor sites simultaneously due to steric hindurence caused due to small distance between them and when the distance between the two donor sites is more, then the ligand now is able to bond with the metal using its multiple sites making it a multidentate ligand.

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When a ligand has multiple donor sites, but uses only say $x$ of them at a particular time, then it's called ambidentate ligand. However, if it uses different number of sites in different compounds (say $x$ in one and $y$ in the other where $x\ne y$), then flexidentate is a more appropriate term as said in the other answer. But this doesn't mean all ambidentate ligands are monodentate.

For example, dithiooxalate is bidentate and ambidentate:

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This example is from the book Concise Inorganic Chemistry by J.D. Lee (Adapted by Sudarsan Guha).

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  • $\begingroup$ I guess you should probably avoid mentioning that flexidentate tidbit, because I couldn't find an official definition for the same on the IUPAC website, so the lack of a credible source might make it seem questionable $\endgroup$ – Yusuf Hasan May 22 at 13:32
  • $\begingroup$ @YusufHasan: Yes. You're right that the IUPAC Gold book doesn't give a definition for 'flexidentate' ligands. But some papers (for example, 1, 2) use this term and I think its meaning is also obvious from its name. However, if enough opposition arises, I'll definitely remove that piece of text from my answer. Thanks! $\endgroup$ – Guru Vishnu May 22 at 13:47
  • $\begingroup$ Other 'credible' sources can be found using this query. $\endgroup$ – Guru Vishnu May 22 at 13:48

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